Although not omposed of karstic Edwards Limestone nor physiographically part of Ecoregion 30, Ecoregion 31b contains springs and streams that show some similarities to those of the Edwards Plateau (30) because they flow over a chalky substrate and likely originate from cool water aquifers beneath the Edwards Plateau.

Active, barren, or stabilized Pleistocene Sand Dunes are found along, and usually to the north of, most major rivers in the Central Great Plains (27); they lack well developed drainage networks and are physiographically and lithologically unlike the rest of Ecoregion 27.